There are known many methods of catalytically converting hydrocarbon starting materials containing olefins using catalysts containing zeolite, and there are many reports on the method of producing ethylene and propylene by catalytic conversion.
JP-A-49-41322 discloses a method of conversion of a paraffin, olefin and/or cycloparaffin (naphthene) of 5 or more carbon atoms, to an aromatic hydrocarbon, ethylene and propylene using an H (proton) type ZSM-5 zeolite. However, according to this method, aromatic hydrocarbons are obtained in a relatively high yield, whereas yields of ethylene and propylene are low.
JP-A-50-49233 discloses a method of conversion of an olefin or paraffin of 2-4 carbon atoms to an aromatic hydrocarbon, ethylene and propylene using a proton type ZSM-5 zeolite. However, according to this method, the aromatic hydrocarbon is also obtained in a relatively high yield, but yields of ethylene and propylene are low.
U.S. Pat. Nos. 4,527,001 and 4,613,721 disclose methods of conversion of butene to ethylene and propylene using an aluminophosphate-based molecular sieve. However, yields of ethylene and propylene are also low in this method.
EPC Publication 0109060 discloses a method of conversion of an olefin of 4-12 carbon atoms to ethylene and propylene using an H type ZSM-5 zeolite having a molar ratio SiO2/Al2O3 of 350 or higher under specific reaction conditions.
WO 2000/010948 discloses a method of conversion of an olefin of 4-12 carbon atoms to ethylene and propylene using a non-proton type ZSM-5 zeolite having a molar ratio SiO2/Al2O3 of 200-5000 and containing a metal of Group IB.
In the method of converting an olefin of 4-12 carbon atoms to ethylene and propylene using a zeolite-containing catalyst, olefins of about 4-8 carbon atoms are obtained as reaction products in addition to ethylene and propylene. This is because the starting olefin is dimerized and decomposed by the catalyst to result in conversion to the composition which is close to the equilibrium composition under the reaction conditions. Therefore, in order to efficiently convert the starting olefin to ethylene and propylene, it is essential to efficiently recycle olefins of 4 or more carbon atoms in the reaction products to the reaction vessel by a simple method.
EPC Publication 0109060 describes a method of recycling to the reaction vessel the olefins of 4-8 carbon atoms obtained by removing aromatic hydrocarbons from the reaction products. Furthermore, WO 2000/010948 describes a method of recycling to the reaction vessel the olefins of 4-8 carbon atoms obtained by removing fractions having a boiling point higher than those of aromatic hydrocarbons of 8 carbon atoms from the reaction products. However, these methods require a plurality of separators for obtaining the recycling materials, which increases apparatus cost and operating cost. Thus, a simpler method has been demanded.